1. Field of the Invention
The invention relates to a control device that switches operation states of two disconnect mechanisms in accordance with a traveling state of a 4WD vehicle. The two disconnect mechanisms are two disconnection mechanisms that are disposed on a driving force source side and an auxiliary driving wheels side of a driving force transmission shaft, respectively.
2. Description of Related Art
4WD vehicles that are provided with two disconnection mechanisms as disconnect mechanisms are widely known. The two disconnection mechanisms are disposed on a driving force source side and an auxiliary driving wheels side of a driving force transmission shaft (for example, propeller shaft), respectively. The driving force transmission shaft is configured to transmit power of the driving force source during 4WD traveling to the auxiliary driving wheels side becoming driven wheels during 2WD traveling. One of the examples is a vehicle that is described in Japanese Patent Application Publication No. 2010-100280 (JP 2010-100280 A). In this 4WD vehicle, the two disconnection mechanisms are released during 2WD traveling. Accordingly, the rotation of the driving force transmission shaft and the like between the two disconnection mechanisms can be stopped. In this case, fuel efficiency can be improved compared to a case where only one of the two disconnection mechanisms is released during 2WD traveling. In addition, various techniques have been proposed with regard to control for transition from 2WD traveling to 4WD traveling in the 4WD vehicle. In JP 2010-100280 A, for example, control is performed so that a torque-variable multi-plate clutch (corresponding to one of the two disconnection mechanisms) is engaged. Accordingly, the rotation of a torque transmission section (corresponding to the rotation of the propeller shaft) is increased in accordance with a predetermined velocity gradient. JP 2010-100280 A discloses a technique for engaging a dog clutch when a relative rotation speed of the dog clutch (corresponding to the other one of the two disconnection mechanisms) is substantially synchronized. In addition, JP 2010-100280 A discloses a technique for learning-correct the torque of the multi-plate clutch during the engagement by comparing the acceleration of the rotation of the torque transmission section to a desired velocity gradient.